Return to zero and sampling pulse generating circuits and method for direct digital up conversion

ABSTRACT

Direct up-conversion of a signal is accomplished using a sampling pulse generator circuit and a gated differential amplifier, enabled by the sampling signal. When not enabled, the output of the differential amplifier is pulled to zero. The sampling pulse is generated from a base frequency sine wave which is squared with a limiting amplifier, and further passed through one or more frequency doublers producing a times two signal, a times four signal and so on. The squared base frequency and frequency doubled signals are logically ORed to produce a short duration pulse which repeats at the frequency of the base signal. The resulting output is an amplitude modulated pulse doublet time domain waveform.

TECHNICAL FIELD

The present invention relates to signal transmission, and moreparticularly, relates to direct up-conversion analog signals.

BACKGROUND INFORMATION

In the field of radar, radio and other signal transmission applications,it is often desirable to up-convert a baseband signal from one frequencyto a higher frequency. Typically, this has been done using multiplelocal oscillators with associated filters, mixers, intermediatefrequency amplifiers and phase-locked loop circuitry. Such circuitsconsume power, are inherently lossy, and can emit spurious unwantedharmonic signals. These unwanted emissions can, with appropriateequipment, be detected and hence reduce the stealth capabilities of suchapplications. Additionally, a number of components associated with theselocal oscillators cannot readily be implemented in integrated circuits,requiring off-chip circuit elements such as crystals and inductors.

When utilizing field portable power sources, it is desirable that powerconsumption be minimized, and power be utilized efficiently. Reducingthe circuit element count in a circuit can reduce power consumption, bututilizing traditional local oscillators and mixers for up-conversion hasa practical limitation in traditional local oscillator-based designs forup conversion.

What is needed is an efficient (i.e. low circuit element count and powerconsumption) way to provide up-conversion, while minimizing spuriousemissions to improve the stealth characteristics of the application.

SUMMARY OF THE INVENTION

The present invention addresses the problems associated with using localoscillators by performing direct up-conversion using a novel combinationof analog and digital circuitry to produce a sampling pulse, which isused to control a gated differential amplifier. This results in a timedomain waveform that is a pulse doublet train which is amplitudemodulated by the input signal to the differential amplifier. The pulsecircuit is generated by frequency doubling, amplifying and limiting (tosquare up the resulting signal) a base signal one or more times, andusing the resulting signals (times two, times four and so forth) toproduce a short duration sampling signal repeating at the frequency ofthe base signal.

In one aspect, the invention includes a system and method which squaresan input sine wave, applies it to a frequency doubler and limitingamplifier (FDLA), and the two signals are fed to a logical NOR gate toproduce a narrow sampling pulse. The sampling pulse is used to controlthe output of a gated differential amplifier. When the sampling pulse isasserted, the output of the gated differential amplifier tracks theinput (from, in one aspect, a digital to analog converter) and when thesampling pulse is not asserted, the output of the differential amplifieris pulled to zero. In another aspect, the doubled input signal is inturn applied to a FDLA to produce a times four signal which is alsoinput to the logical NOR gate, in which aspect the sampling pulse isrepeated at the frequency of the input sine wave, but has a duration ofa single half cycle of eight times the frequency of the input sine wave.

In another aspect, the invention is, at least in part, implemented as aMonolithic Microwave Integrated Circuit (MMIC), and in yet anotheraspect uses pseudomorphic high electron mobility transistors (PHEMT). Instill another aspect, the differential amplifier is controlled by gatingthe biasing current to the amplifier. The output of the differentialamplifier, in another aspect, is converted to unbalanced microstripusing a planar balun.

An advantage of the present invention is that it may be implemented in asingle monolithic integrated circuit without the need for external localoscillators and mixers. It is also an advantage in a wideband systemthat eliminating the need for local oscillators reduces the potentialfor in-band and near in-band re-radiation. Yet another advantage is areduced number of circuit elements which reduces cost and powerconsumption.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bebetter understood by reading the following detailed description, takentogether with the drawings wherein:

FIG. 1 is a block diagram of a return to zero (RTZ) circuit and samplingpulse generator in accordance with the present invention;

FIG. 2 is a time domain plot of a sample output waveform of a direct upconversion;

FIG. 3 is a plot of an output spectrum from a sample waveform afterfiltering by X-Band pre-selector filter;

FIG. 4 is a block diagram of an implementation of an RTZ circuit in amicrochip; and

FIG. 5 is a block diagram of a clock generation circuit implementationin a microchip in accordance with one feature of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Direct up conversion of analog signals is of interest to radio, radarand other transmitting systems. Direct up-conversion can provide size,power, weight and cost benefits compared to analog up conversion usinglocal oscillators and mixers. Elimination of local oscillators reducesthe potential for in-band and near in-band local oscillatorre-radiation, which is desirable in stealthy applications.

A direct up-conversion application can be segmented into two sections: areturn to zero (RTZ) sampler and a sampling pulse generator. Animplementation of a return to zero (RTZ) and sampling pulse circuit inaccordance with the principles of the present invention is describedbelow.

The RTZ uses a gated differential amplifier that tracks an input signalfrom an analog to digital converter at the input during a short samplinginterval, and otherwise outputs zero volts.

FIG. 1 shows a block diagram of an RTZ circuit and sampling pulsegenerator in accordance with the present invention. A sine wave input 10is converted to a square wave by a limiting amplifier 12, producing abase pulse stream 14. The base pulse stream 14 is fed to a frequencydoubler and limiting amplifier (FDLA) 16 producing a times two (×2)pulse stream 18 at twice the frequency of the first pulse stream 14. Inturn, the times two pulse stream 18 is fed to a second FDLA 20 producinga times four (4×) pulse stream 22.

The three pulse streams (base 14, times two 18 and times four 22) areapplied as inputs to a logical NOR circuit 24. The result is a samplingpulse output 25 which is a narrow pulse having a pulse width of a singlehalf cycle of the times four pulse stream 22 and a frequency rate equalto the rate of the base pulse stream 14.

It has been found to be better to derive the sampling pulse by frequencydoubling rather than starting with a higher frequency and dividing down.Frequency doubling provides greater stability and repeatability.

The output 25 of the logical NOR circuit 24 is applied to a controlinput 28 of a gated differential amplifier 26. When the control input 28of the gated differential amplifier 26 is enabled, it the output of thedifferential amplifier 26 follows the input 27. In one implementation,the input 27 is connected to the output of a digital to analogconverter.

When the control input 28 of the gated differential amplifier 26 isdisabled, the current source (not shown) to the gated differentialamplifier 26 is disabled, and the output 34 is pulled to zero by aswitch 32.

In a preferred embodiment, the RTZ and sampling pulse generator areimplemented as a monolithic microwave integrated circuit (MMIC) withpseudomorphic high electron mobility transistors (PHEMT) fabricatedusing a 0.15 double recess fully selective (etch stop) process. Thisprocess has been found to be versatile, allowing the combination of lownoise, power, passive switching and mixed signal devices on the samemask set. The use of etch stop results in good uniformity and highyield. The MMIC is implemented on a 101.6 um thick substrate with slotvias and incorporated 25 ohms/square TaN (Tantalum Nitride) resistorsand 400 pF/mm² metal-insulator-metal (MIM) SiN (Silicon Nitride)capacitors.

Referring to FIG. 2, a time domain plot 40 of the output 34 of an RTZcircuit fabricated according to the teachings of the present inventionis shown. A 100 MHz reference signal from a digital to analog converter(not shown) was supplied to the input of the gated differentialamplifier 26, and a 2 GHz sine wave (not shown) was supplied as theinput 10 to the sampling pulse generator. Other frequencies could beused advantageously. The X axis 42 is 1 nanosecond per division, and theY axis 44 is 10 mV per division. The plot 46 shows the measuredamplitude modulated pulse doublet time domain waveform 46 recorded usinga sampling scope.

FIG. 3 shows the output spectrum 50 of the modulated pulse doubletwaveform 46 after passing it through an X-Band pre-selector filter (notshown). The X axis 56 is harmonic frequency at 5.0 GHz per division. TheY axis 58 is ten dB per division. Two modulation sidebands 52 and 54 areclearly evident. The fifth harmonic of 2 GHz is shown suppressed byeighteen dB, and the second harmonic (62 and 64) of the modulatingfrequency is suppressed by forty dB

FIG. 4 shows a schematic diagram of an RTZ circuit 70 as implemented inan MMIC. The gated differential amplifier 26 has a control input 28which when asserted switches the control transistor 74 to supply currentto the differential amplifier 26. When the control input 28 is notasserted, pull down transistors 76 and 78 pull the output of thedifferential amplifier 26 to zero. The output of the differentialamplifier 26 was converted to unbalanced microstrip using a planar balun(not shown).

FIG. 5 shows a schematic of a sample pulse generator circuit. A sinewave input 10 is applied to a circuit section 90 which squares the inputsine wave to produce the base pulse stream 14, and produces the timestwo pulse stream 18 and the times four pulse stream 22. The NOR circuit24 produces the sampling pulse 25. Circuit section 94 supplies areference voltage to the NOR circuit.

Modifications and substitutions by one of ordinary skill in the art areconsidered to be within the scope of the present invention, which is notto be limited except by the following claims. For example, discretecomponents rather than integrated circuits could be used. Differentfrequencies than those shown could be used. More than two FDLA's couldbe used to make shorter sampling pulses.

What is claimed is:
 1. A method comprising: in an electronic circuit,applying at least a first FDLA to a first pulse stream to derive asecond pulse stream; deriving a sampling pulse stream from a logical NORof at least the first and second pulse streams; using the sampling pulsestream to enable an output of a gated differential amplifier.
 2. Themethod of claim 1 further comprising deriving a third pulse stream byapplying a second FDLA to the second pulse stream and wherein the inputto the logical NOR further comprises the third pulse stream.
 3. Themethod of claim 1 wherein an input to the gated differential amplifieris an output from a digital to analog converter.
 4. The method of claim1 wherein the sampling pulse stream enables a pull-down circuit appliedto the output of the gated differential amplifier.
 5. The method ofclaim 1 wherein the circuit is at least partially implemented in amicrochip.
 6. The method of claim 5 wherein the circuit comprises atleast one pseudomorphic high electron mobility transistor.
 7. The methodof claim 6 wherein the microchip comprises a monolithic microwaveintegrated circuit.
 8. The method of claim 6 further comprisingconverting the output of the gated differential amplifier to unbalancedmicrostrip using a planar balun.
 9. The method of claim 1 wherein thefirst pulse stream is a square wave derived from applying a limitingamplifier to a sine wave.
 10. The method of claim 3 wherein the samplingpulse stream controls a current source biasing the differentialamplifier.
 11. A circuit comprising: at least a first pulse stream; atleast a first FDLA receiving as an input the at least first pulse streamand an output providing a second pulse stream; a logical NOR circuitreceiving as input the at least the first and second pulse streams andproviding a sampling pulse stream as an output; a gated differentialamplifier having the sampling pulse stream as a control input.
 12. Thecircuit of claim 11 further comprising a second FDLA receiving as aninput the second pulse stream and providing as an output a third pulsestream, and wherein the input to the logical NOR circuit furthercomprises the third pulse stream.
 13. The circuit of claim 11 wherein aninput to the gated differential amplifier is an output from a digital toanalog converter.
 14. The circuit of claim 11 wherein the sampling pulsestream in a first logical state enables a pull-down circuit connected tothe output of the gated differential amplifier.
 15. The circuit of claim11 wherein the circuit is at least partially implemented as a microchip.16. The circuit of claim 15 wherein the circuit comprises at least onepseudo-morphic high electron mobility transistor.
 17. The circuit ofclaim 16 wherein the microchip comprises a monolithic microwaveintegrated circuit.
 18. The circuit of claim 17 further comprising abalun connected to the output of the gated differential amplifier toconvert the output to unbalanced microstrip.
 19. The circuit of claim 11further comprising a limiting amplifier having a sine wave as an inputand the first pulse signal as an output.
 20. The circuit of claim 11wherein the sampling pulse stream controls a current source biasing thedifferential amplifier.
 21. A system comprising: an electrical circuitcomprising at least a first FDLA having an input and an output, furthercomprising a logical NOR circuit and a gated differential amplifier;wherein a first pulse train applied to the input of the at least firstFDLA produces a second pulse train at the output of the at least firstFDLA; and wherein at least the first and second pulse trains compriseinputs to the logical NOR circuit and wherein the output of the logicalNOR circuit comprises a control signal; a return-to-zero circuitcomprising a gated differential amplifier having an input and an outputand a control input connected to the control signal, wherein the outputof the gated differential amplifier follows the input of the gateddifferential amplifier when the control signal is in a first logicalstate, and wherein the output of the gated differential amplifier ispulled toward zero when the control signal is in a second logical state.22. The system of claim 21 within the second FDLA having the secondpulse stream as an input and a third pulse stream as an output, andwherein the input to the logical NOR circuit further comprises the thirdpulse stream.
 23. The system of claim 21 wherein an input to the gateddifferential amplifier is an output from a digital to analog converter.24. The system of claim 21 wherein the sampling pulse stream in a firstlogical state enables a pull-down circuit applied to the output of thegated differential amplifier.
 25. The system of claim 21 furthercomprising a balun connected to the output of the gated differentialamplifier to convert the output to unbalanced microstrip.
 26. The systemof claim 21 wherein the first pulse stream comprises a square wavederived from applying a limiting amplifier to a sine wave.
 27. Thesystem of claim 21 wherein the sampling pulse signal controls a currentsource biasing the differential amplifier.